High current, low emittance electron beams are required for a number of applications in scientific research, technology, and medicine. Applications such as nuclear physics, radiography, radiotherapy, and laser technology require electron beams with energies of 10-100 MeV, instantaneous peak currents of 10-100 amperes, and normalized current densities in excess of 1.times.10.sup.6 amp/cm.sup.2. Electron linear accelerators are in principle capable of providing such beams, but the beam quality is typically limited by space charge and phase-dependent focusing forces in the electron gun that injects the electrons into the accelerator waveguide.
The most common type of electron gun suitable for use with a linear accelerator subjects electrons emitted from a thermionic cathode to a DC electric field for initial acceleration, and then passes the electrons through a longitudinal microwave electric field to provide a bunched beam suitable for injection into the accelerator. Typical electron guns are designed to provide electron energies on the order of 100 keV.
In conventional DC guns, the electrons are subject to non-linear space charge forces which increase the emittance. These space charge forces result from non-uniformities in the charge density of the beam. More particularly, where the charge density is a function of radius or azimuth, the electrons are subject to non-linear defocusing forces which increase the apparent beam radius and angular divergence, and hence the emittance. While it is in principle possible to minimize the emittance growth due to space charge by suitably shaping the electrodes surrounding the cathode, such a technique depends on uniform emission from the cathode. In practice, the emission is never uniform, and the electrode configuration can provide only a partial and imperfect correction to the space-charge induced emittance growth.
It is possible to reduce such space charge forces by operating the gun at a higher accelerating DC field to increase the electrons' energy. However, for pulsed or DC fields greater than about 100 kv/cm, electron (and possibly ion) emission from the electrodes and insulating elements grows rapidly, leading to internal arcs.
Conventional DC guns also subject the electrons to phase-dependant focusing forces which further increase the emittance. As alluded to above, the DC electron beam must be velocity modulated by passage through a buncher cavity to form the bunched structure required for injection into the linear accelerator. However, the longitudinal accelerating field in the bunching cavity generates an azimuthal magnetic field which focuses or defocuses the electrons, depending on the phase at which they pass through the cavity. Such focusing force typically reaches its maximum value as the longitudinal electric field passes through zero. Thus, electrons entering the bunching cavity in a continuous stream exit with a range of transverse momenta corresponding to their initial radial displacement and their phase at the time of passage through the cavity. For a randomly-phased initial electron beam, the range of transverse momenta induced by the microwave field increases the spread in divergence angles of the electrons in the beam, and hence the emittance.
The two-mile accelerator at the Stanford Linear Accelerator Center ("SLAC"), Stanford, Calif., is an example of a machine whose beam is characterized by high emittance caused by the inherent properties of the electron gun system. Until recently, the high emittance did not impose a fundamental constraint on the research program, since there was no need to focus the electron beam to an extremely small spot. However, the beam properties have been found unsuitable for the linear collider program now underway at SLAC. The solution developed at SLAC is based on the use of synchrotron radiation damping in a specially designed storage ring. The electron beam is extracted from the main beam line at an energy of 1 GeV, injected into the storage ring where it is allowed to circulate for about 10 milliseconds, and re-injected into the accelerator for acceleration to the final energy. The synchrotron radiation emitted by the electrons in the storage ring monochromatizes the beam and reduces its transverse dimensions, thereby making the beam suitable for use in the linear collider. Unfortunately, the cost of this solution to the beam emittance problem is estimated to be about $15,000,000.
Thus, existing gun technology is ill-suited to produce low emittance, high current electron beams due to the non-linear space charge forces and phase-dependent focusing forces which are intrinsic to their operation. Moreover, even where the existing electron gun and buncher cavity technology is satisfactory, the power supplies, microwave cavities, and focusing magnets required for the operation of such systems are large and expensive, and reduce the overall reliability of the system.